The synthesis of nebularine and its analogs via oxidative desulfuration in aqueous nitric acid

Article abstract of DOI:10.1080/10426507.2016.1225057

The synthesis of nebularine and its analogs has been achieved via oxidative desulfuration in H₂O for the first time. With 50% HNO₃as an oxidant and solvent, 18 products were obtained in good yields (70%–94%). The oxidative desulfuration system could tolerate different functional groups including fluoro, chloro, amino, alkyl, allyl, ribosyl, deoxyribosyl, and arabinofuranosyl groups.More importantly, the drug nebularine could be obtained successfully on a 20 g scale, which made this route more attractive for industrial applications.

Full text of DOI:10.1080/10426507.2016.1225057

Phosphorus, Sulfur, and Silicon and the Related Elements
ISSN: 1042-6507 (Print) 1563-5325 (Online) Journal homepage: http://www.tandfonline.com/loi/gpss20
The Synthesis of Nebularine and Its Analogues via
Oxidative Desulfuration in Aqueous Nitric acid
Ran Xia, Li-Ping Sun & Gui-Rong Qu
To cite this article: Ran Xia, Li-Ping Sun & Gui-Rong Qu (2016): The Synthesis of Nebularine
and Its Analogues via Oxidative Desulfuration in Aqueous Nitric acid, Phosphorus, Sulfur, and
Silicon and the Related Elements, DOI: 10.1080/10426507.2016.1225057
To link to this article: http://dx.doi.org/10.1080/10426507.2016.1225057
Accepted author version posted online: 19
Aug 2016.
Published online: 19 Aug 2016.
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Date: 22 August 2016, At: 07:16

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The Synthesis of Nebularine and Its Analogues via Oxidative Desulfuration in Aqueous
Nitric acid
Ran Xiaa,*, Li-Ping Suna and Gui-Rong Qub
aSchool of Chemistry and Chemical Engineering, School of Life Science and Technology,
Xinxiang University, Eastern Jinsui Road, Xinxiang City, Henan Province, 453003, China
bSchool of Chemistry and Chemical Engineering, Henan Normal University, 46#, Eastern
Jianshe Road, Xinxiang City, Henan Province, 453007, China
Corresponding author Email : ranxia518@hotmail.com
ABSTRCT
The synthesis of nebularine and its analogues has been achieved via oxidative desulfuration in
H2O for the first time. With 50 % HNO3 as an oxidant and solvent, 18 products were obtained
in good yields (70–94%). The oxidative desulfuration system could tolerate different
functional groups including fluoro, chloro, amino, alkyl, allyl, ribosyl, deoxyribosyl and
arabinofuranosyl groups. More importantly, the drug nebularine could be obtained
successfully on a 20 g scale, which made this route more attractive for industrial applications.
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KEYWORDS
nebularine; desulfuration; base modification; nucleoside synthesis
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INTRODUCTION
Sulfur chemistry plays an important role in organic chemistry,1 and great efforts have
been devoted to the synthesis of sulfur substituted compounds (R-SH) starting from amines
(R-NH2),2 haloalkanes (R-X)3 and others.4 In comparison, the desulfuration reaction has been
less studied. Most commonly, the desulfuration reaction must be carried out by
nickel-catalyzed hydrogenolysis.5 Metal-free desulfuration in H2O has not been explored to
date.
Nebularine is a naturally occurring purine riboside antibiotic isolated from the fungus
Lepista nebularis,6 which inhibits mouse Sarcoma 180 and mycobacteria, and can act as a
universal base to bind with all four of the nucleosides of DNA.7 The analogues of nebularine
Scheme 1) also play a significant role in the pharmaceutical industry. For example,
famciclovir is a clinical drug approved by FDA for the treatment of herpes zoster and acute
recurrent genital herpes.8 Emapunil has been ever a Novartis’s anxiolytic bioactive compound
under clinical phase II.9 Besifovir is developed by Roche to treat HBV and now is under
clinical phase III.10 Furthermore, others and our group also found that nebularine and its
analogues could also be used as key building blocks in the synthesis of complex nucleoside
molecules via the late-stage modification.11-12 Consequently, the development of efficient
methods for the synthesis of nebularine and its analogues has stimulated considerable interest.
Although research has been conducted for the synthesis of nebularine and its analogues,
efficient and practical methods still remain particularly challenging to chemists. Early studies
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in this field relied on the Pd/C catalyzed hydrogenolysis reduction of 6-chloropurines or
6-akylthiopurines.13-14 Recently, our group demonstrated the CuSO4 catalyzed
dehydrazination of 6-hydrazinopurine nucleosides.15 However, these two methods suffered
from an excess amount of metal catalysts which were difficult to deal with and brought health
threat and serious environmental pollution. (Scheme 2). In addition, the starting materials
must be obtained through multistep synthesis from inosine derivatives.16 The
diazotization/deamination from more available adenosine derivatives invented by Robins was
in low yield (31%) due to forming inosine as a side product.17 To date, no better alternatives
have been developed.
6-Mercaptopurine nucleoside (1a) could be prepared in bulk from commercially
available inosine through simple chemistry.18 In the context of ongoing projects on the
synthesis of nucleosides,11, 15, 19 herein we describe a mild metal-free method for the synthesis
of nebularine and its analogues via the HNO3 oxidized desulfuration in H2O.
RESULTS AND DISCUSSION
Initially, we started our study using 1a as model substrate to optimize the reaction
conditions (Table 1). When concentrated HNO3 (69%) was used as the oxidant and solvent at
ambient temperature for 0.5 h, nebularine (2a) was isolated in 32% yield (Entry 1). Increasing
o
the temperature to 50 C, nebularine could be obtained with slightly increased yield of 43%
(Entry 2). To our delight, 74% and 82% yields of nebularine were obtained when decreasing
the HNO3 concentration to 50%-60% which was more acceptable for handling (Entries 4-5).
Shortening the reaction time and continuing decreasing the concentration were unbeneficial to
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the yields (Entries 7-8). Therefore, the optimal reaction condition is 50% HNO3 at 50 oC for 1
h.
To evaluate the generality of the oxidative desulfuration reaction, a number of
6-thiopurine derivatives with different substituents at N9 such as ribosyl, deoxyribosyl,
arabinofuranosyl, benzyl, alkyl or allyl groups were subjected to the optimized conditions
(1a-1l). As shown in Table 2, the corresponding oxidative desulfuration 6-H purine
derivatives were obtained in good to excellent yields (2a-2l, 71%–95%). Compared to the
sugar substituted substrates, the benzyl, alkyl and allyl substituted ones gave higher yields
(2a–2d vs. 2e-2k). When a H atom was substituted with a chloro or fluoro atom at C2 in the
purine, the yields of the products were slightly enhanced (2b vs. 2c, 2e vs. 2f, 2h vs. 2i).
When a H atom at C2 in the purine was substituted with an amino group, the lower yield
could be observed (2e vs. 2g, 2h vs. 2j). These results indicated that the oxidative
desulfuration system could tolerate different functional groups successfully. The free
nucleobase could also proceed well through the oxidative desulfuration reaction, giving the
corresponding product in a good yield (2l). The application of this reaction on 20 g scale and
nebularine was obtained in pure form, without need for chromatographic purification.
Next, a variety of thio-substituted purines and other heteroaromatics were examined
(Table 3, 4a-4f, 70%-82%).20-21 The heteroaromatics such as triazole (3a), benzothiazole (3c)
and pyrimidine (3d) displayed good reaction activities. It was worth noting that phenylthiol
bearing electron-withdrawing groups could be desulfuratated in good yield (4b, 70%).
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However, the unsubstituted or mononitrosubstituted phenylthiol only gave trace of yields. The
desulfuration of 8-thiopurine or 2-thiopurine nucleosides also gave rise to the corresponding
products in good yields (4e and 4f, 79% and 76% respectively).
Throughout the reaction, SO2 gas was observed from the mixture. The HPLC-MS
showed a peak 441.06 which should be the intermediate 5. Based on these results, a
preliminary mechanism for the desulfuration reaction is proposed in Scheme 3: 1) The thio
group was oxidized to sulfinic acid; 2) The sulfinic acid underwent spontaneous
decomposition leading to SO2 evolution. In this process, the acetyl groups in sugar were
removed. The optimization of conditions also supported this proposal, because heating was
beneficial to accelerate decomposition of sulfinic acid.
CONCLUSION
In summary, we realized the metal-free oxidative desulfuration reaction for the synthesis
of the natural occurring nucleoside nebularine and its analogues. With cheap aqueous HNO3
solution as an oxidant and solvent, the synthesis of a series of nebularine and its analogues
proceeded well in good to excellent yields. The oxidative desulfuration system could tolerate
different functional groups including fluoro, chloro, amino, alkyl, allyl, ribosyl, deoxyribosyl
and arabinofuranosyl groups. More importantly, the drug nebularine could be obtained
successfully even on a 20 g scale, which made this route more attractive for industrial
application. This new and metal-free method, avoiding the use of toxic metals, represents a
promising route to nebularine and its analogues. Further studies into the mechanism are
currently underway.
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EXPERIMENTAL
Materials and Methods
Melting points were recorded with a micro melting point apparatus and were
1
uncorrected. NMR spectra were recorded with a 400 MHz spectrometer for H NMR, 100
MHz for 13C NMR. Chemical shifts δ are given in ppm relative to tetramethylsilane as
1
13
internal standard, residual CHCl3 for H or CDCl3 in C NMR spectroscopy. Multiplicities
are reported as follows: singlet (s), doublet (d), doublet of doublets (dd), triplet (t), quartet (q),
multiplet (m). High resolution mass spectra were taken with a 3000 mass spectrometer, using
Waters Q-TofMS/MS system. For column chromatography silica gel (200-300 mesh) was
used as the stationary phase. All reactions were monitored by thin layer chromatography
(TLC). All reagents and solvents were purchased from commercial sources and purified
commonly before used. The Supplemental Materials contains complete characterization data
and 1H and 13C NMR spectra for products 2 and 4 (Figures S 1 – S 36)
Typical Procedure for oxidative desulfuration reaction with HNO3
1a (0.142 g, 0.5 mmol) was slowly added to a stirred solution of HNO3 (50%, 5 mL).
o
The mixture was heated to 50 C and kept at this temperature for 1 h. The solvent was
neutralized by 1 mol/L NaOH and was concentrated in vacuo. The salts were filtered and
washed with EtOH (5 mL). The mother liquor was evaporated to dryness and was
recrystallized from H2O to give 2a as a white solid (0.103 g, 82% yield).
ACKNOWLEDGEMENT
We thank the National Natural Science Foundation of China (21172059), Key Scientific
Research Projects of Henan Province (16A150042, 16A180035), Key scientific and
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technological projects of Xinxiang City (CXGG16037) and the Science and Technology
Innovation Fund of Xinxiang University (15ZP06) for financial support.
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Table 1 The influence of HNO3 concentration, reaction temperature and reaction time on the
yields of nebularine a
Entry HNO3/%
Temperature/℃
Reaction time/h
Yield/% b
69
69
69
60
50
50
40
25
50
50
50
50
50
50
0.5
0.5
1
32
43
61
74
82
78
72
1
2
3
4
5
7
6
1
1
1.5
1
a Reaction conditions: 1a (0.5 mmol), HNO3 (5 mL); b Isolated yield based on 1a.
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Table 2 The oxidative desulfuration reaction of 6-thiopurine derivatives
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Table 3 The oxidative desulfuration reaction of thio-substituted aromatic compounds
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Scheme 1 Nebularine and its analogues used as drugs.
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Scheme 2 The synthetic routes to nebularine.
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Scheme 3 Potential mechanism of the desulfuration reaction
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